The present invention relates to wireless communications, and more particularly to the sensing and protecting of wireless transmissions from a user of a spectral resource.
The radio spectrum is a limited resource that should be shared between many different types of equipment such as cellular, home network, broadcast, and military communication equipment. Historically, each part of the radio spectrum has been allocated (e.g., in a country- or region-wide basis) to a certain use (called a “licensed” and/or “primary” use), such as only for television (“TV”) or only for particular types of wireless communications. This strategy has resulted in all applications/uses being disallowed on the allocated carrier frequency except for those applications included in the license agreement.
There are clear advantages to using dedicated spectrum for wireless communications at least in that, because the frequency band in question is reserved, no interference from other systems should occur. This yields predictable network capacity and quality of service.
However, in practice, the dedication of portions of the radio spectrum to one or only a few types of users results in large parts of the radio spectrum being unused much of the time. For instance, in the Ultra-High Frequency (UHF) band, where TV broadcasts take place, large geographical areas are unused, mainly due to the large output power needed for such applications; this large output power compels a large reuse distance in order to minimize the risk of interference. An example of such geographical areas within Scandinavia is illustrated in FIG. 1. In FIG. 1, the shaded areas represent geographic locations in which a given carrier frequency is being used by a licensed user (e.g., by Broadcast TV). In the remaining areas, the so-called “white spaces”, the given carrier frequency is allocated to the licensed user but is not actually being used by that user.
In order to make better use of the licensed spectral resources, some countries will, in the future, allow unlicensed services (so called “secondary” uses) to take place in areas (called “white spaces”) in which the licensed (primary or “incumbent”) user is not transmitting. However the primary/incumbent user will always have priority for the use of the spectrum to the exclusion of others. Therefore, some sort of mechanism needs to be in place to ensure that there is only a low probability that the unlicensed users are causing interference to the licensed user.
One mechanism is to install the unlicensed network in a geographical area where at least some parts of the licensed spectra are known to be unused.
However, even more use of the white space can be made if the non-interference mechanism adopts a detection strategy in which it operates on the licensed frequency (or frequencies) in the white space only so long as no licensed user transmissions are detected, and ceases such operation as soon as licensed user transmissions are detected. In this context, ceasing operation may mean ceasing all operation, or alternatively may mean ceasing operation only on those frequencies that are detected as being “in use”, and otherwise continuing to operate on other frequencies in the white space. The most straightforward sensor is a signature detector adapted to detect specific signatures transmitted from the licensed/primary user (typically implemented as a matched filer). An example of a white space system currently being standardized is IEEE 802.22. An overview of this system can be found in Cordeiro et al, “IEEE 802.22: An introduction to the First Wireless Standard based on Cognitive Radios”, Journal of Communications, Vol 1, No 1, April 2006.
In commercial embodiments, the higher cost of signature detectors may make them unfeasible. As a less expensive alternative, sensors can be implemented to function as received power detectors. These essentially compare a received power level on a white space given frequency and compare this with a threshold level. So long as the received power level is below the threshold power level, the incumbent equipment can be considered to not be in use.
Both of the previously described approaches of ascertaining white space spectrum availability are, in a sense, all-or-nothing approaches. When the lack of interference to the incumbent is ensured by the choice of the geographical location, the white space spectrum utilization is static in its nature. Thus, only the location and frequency band combinations with no activity at any time are considered, which may be a significant limitation. When the sensor signals are used as spectrum availability indications, the frequency band in question is activated or deactivated in the whole area.
It remains a desirable goal to provide improved methods and apparatuses that allow non-incumbent equipment to operate in a white space area without disturbing operation by incumbent equipment.